This invention relates to a starch-based, alkaline corrugating adhesive composition containing a crosslinking additive which imparts water resistance thereto and is substantially free of formaldehyde.
The procedures employed in the production of corrugated paperboard usually involve a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed therebetween. The above-described procedure produces what is known to those skilled in the art as a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard is desired, in which an inner fluted layer is sandwiched between two facings, a second operation is performed wherein the adhesive is applied to the exposed tips of a single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the use and operation of corrugators in general are described in U.S. Pat. Nos. 2,102,937 and 2,051,025 to Bauer.
Starch-based adhesives are most commonly used in the corrugating process due to their desirable adhesive properties, low cost and ease of preparation.
The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the full-formulation adhesive. In the corrugating process, the adhesive is applied (usually at between 25.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. Such adhesives are described in the above-noted patents to Bauer. Typical "no carrier" starch adhesives are described in U.S. Pat. No. 3,487,033 to McElmury et al. and U.S. Pat. No. 3,355,307 to Schoenberger et al.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water-resistant bonds which can withstand extended exposure to high humidity, liquid water, melting ice and the like. A number of approaches have been devised to produce water-resistant corrugating adhesives. One method involves the preparation of an acidic, starch-based adhesive wherein urea-formaldehyde resin is added to the composition, together with an acidic catalyst such as aluminum sulfate, to produce water-resistant bonds in the corrugated board manufactured therewith. The adhesive composition itself, however, is deficient in other important properties such as corrugator bonding speeds, viscosity stability, and pot life and exhibits excessive formaldehyde odor. In addition, acidic corrugating adhesives tend to be corrosive.
The many disadvantages associated with the acidic corrugating adhesives led to the development of water-resistant alkaline curing starch-based adhesives for use in the corrugating industry. In the preparation thereof, a thermosetting resin, such as, e.g., urea-formaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, phenol-formaldehyde, diacetone acrylamide-formaldehyde, ketone-aldehyde and urea-acetone-formaldehyde condensate, is added to the adhesive as a crosslinking additive for the amylaceous components to produce water-resistant bonds. Preferred among these resins for superior water-resistant properties are ketone-formaldehyde condensates as disclosed in U.S. Pat. No. 2,529,851, and particularly acetone-formaldehyde resins. Some adhesives made from such resins, however, suffer from poor pot life and viscosity instability, as well as considerable formaldehyde odor.
In recent years, due to the toxicity of and increasing governmental regulations concerning formaldehyde, serious efforts have been made to reduce the levels of exposure to formaldehyde in the industrial workplace. Acetone-formaldehyde resins such as are employed as crosslinking additives in corrugating adhesives contain about 1.0 to 4.0% free (unreacted) formaldehyde by weight. Prior attempts to reduce formaldehyde levels in crosslinking additives as taught in U.S. Pat. Nos. 3,019,120 and 3,294,716 have not reduced free formaldehyde amounts to a significant extent and/or have resulted in diminution of the degree of water resistance achieved in the bonds formed.
In U.S. Pat. No. 4,366,275 to Silano et al., the crosslinking additive used with the starch-based alkaline corrugating composition comprises a mixture of acetone-formaldehyde condensate and dimethylol dihydroxy ethylene urea (DMDHEU) wherein at least a portion of the DMDHEU present is produced "in situ" by reaction of the free formaldehyde contained in the acetone-formaldehyde condensate with dihydroxy ethylene urea. The patent discloses that the unreacted formaldehyde in the acetone-formaldehyde resin is reduced to about 0.1 to 2% by weight. Experience has shown, however, that in most instances the free formaldehyde is reduced only to a level of about 0.5 to 0.9% by weight of the condensate. Current industry requirements call for still lower levels of unreacted formaldehyde.
Accordingly, it is an object of the present invention to provide a ketone-formaldehyde crosslinking additive with very low levels of free formaldehyde (less than 0.4%, and preferably less than 0.2%. by weight) which, nevertheless, imparts water-resistance, viscosity stability and acceptable pot life to starch-based alkaline corrugating adhesive compositions.
It is another object to provide a water-resistant, starch-based alkaline adhesive composition containing such crosslinking additive in the manufacture of corrugated paperboard.